Add system_note_icon mapping for epic actions

What does this MR do?

Adds icons to epic system notes related to issue operations (e.g. adding an icon to an epic)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #4262 (closed)